1. Field of the Invention
The invention is directed to a zinc powder or zinc alloy powder for alkaline batteries, which powder has a specific grain size distribution, and to an alkaline battery wherein the zinc powders or zinc alloy powders of the invention are employed as electrode.
2. Description of Related Art
A large number of various alloyed zinc powders for use in alkaline batteries have been described in the prior art. Therein, the zinc powders are alloyed in various ways. Conventionally, zinc powders free of mercury, cadmium and preferably also free of lead are used. When used in alkaline batteries, such zinc powders involve the disadvantage of gradual decomposition with evolution of gas as a result of various reaction processes, thereby adversely affecting the durability, shelf life, as well as the electrical properties of the battery. To prevent this, alloys of zinc powder including minor amounts of other metals have mostly been used for battery purposes. Essentially, indium, bismuth, aluminum, magnesium, and calcium are mentioned in the prior art as alloy elements which reduce battery gassing. When used in batteries, these alloy components are added with the aim of obtaining battery gassing as low as possible, thereby increasing the shelf life and safety of the batteries during use.
However, the technical demands on batteries have increased substantially in recent years. In particular, the demands on batteries and their electrical performance have increased considerably as a result of the enormous expansion of mobile appliances in the sector of digital communication and entertainment electronics, such as digital video cameras, cellular phones, CD players, MP-3 players, lap-tops, etc. As a result, attempts are now being made to improve the electrical properties not only by alloy-technological means, but rather, to achieve such improvements by means of special powder distributions, particle shapes and bulk densities of the employed zinc powders or zinc alloy powders. According to the present state of the art, zinc powders or zinc alloy powders are generally used wherein the particle size spans a wide range of about 32-500 μm, the grain size distribution being random.
Thus, WO 00/74157 A1 describes a mixture of zinc powder or zinc alloy powder and a liquid electrolytic medium, wherein the volume of the medium approximately corresponds to the interstices between the particles in dry bulk. These particles have a bulk density less than 2.8 g/cm3. The purpose of this development is to achieve direct contact between virtually all of the particles in a way that sufficient liquid electrolytic medium is still present to dissolve the metal and/or alloy particles, thereby generating electricity.
Similarly, WO 01/03209 A1 describes a mixture of zinc or zinc alloy particles and a liquid electrolytic medium wherein the bulk density is even lower and is about 2.3 g/cm3. Furthermore, metal or metal alloy particles of irregular shape and irregular surface are employed.
WO 99/07030 A1 describes zinc powders or zinc alloy powders for use in alkaline batteries, said zinc or zinc alloys being admixed with varying amounts of extremely fine particles having a size of 200 mesh or less (corresponding to a particle diameter of ≦74 μm). Such admixture of fine zinc particles has been found to result in an improvement in the electrical properties of the battery, particularly an improvement in the maximum discharge rate. Said rate defines the period of time within which electric voltage can be obtained from a battery under load, with no drop of the battery voltage below a specific value. This quantity is particularly important for uses in the high-power sector, particularly in cellular phones and other electronic products which normally measure the battery voltage and signal the user in case of a voltage drop under load. Frequently, however, this may be the reason for untimely and unnecessary replacement of the batteries despite sufficiently available capacity.
The demands on zinc powders and zinc alloy powders for use in electric batteries vary considerably, and to date, no success has been made in developing a zinc powder or a zinc alloy powder that achieves an improvement in all of the test procedures and, in particular, has low battery gassing. For example, this can also be seen in the WO 99/07030 described above. In the examples described therein, battery gassing of zinc powders and zinc alloy powders suitable for high-power uses has not been investigated, and it must be assumed that the battery properties are not optimal.